1. Field of the Invention
The present invention relates to an optical device having a single optical fiber or a plurality of optical fibers (optical fiber array), or a single optical waveguide or a plurality of optical waveguides, and more particularly to an optical device suitable for monitoring signal light while it is propagated through such an optical transmitting means.
2. Description of the Related Art
When using present optical communication technology, it is important to monitor the communication quality. In particular, monitoring optical output plays an important role in the field of wavelength multiplex communication technology.
In recent years, there have been growing demands for smaller size, higher performance, and lower costs in optical output monitoring technology.
Heretofore, certain technology, for example, as disclosed in Japanese Laid-Open Patent Publication No. 2001-264594, has been proposed. According to such proposed technology, an optical waveguide core is disposed in a substrate, and then a slit is formed in the substrate obliquely across the core (the optical axis thereof). A light reflecting base (filter member) is inserted into the slit.
Of signal light propagated through the optical fiber, a light component (reflected light) reflected by the light reflecting base is extracted out of the optical waveguide. The reflected light is detected by a photodetector, for example, to monitor the signal light.
According to the conventional arrangement, as shown in FIGS. 9 and 10, when a filter member 206 is inserted in a slit 204 extending across a core 202 of an optical waveguide 200, an upper portion (hereinafter referred to as an upper portion of the filter member 206), including an upper end 208a of a face surface (light-incident surface 208) of the filter member 206, and an upper end 210a of a reverse surface (light-exiting surface 210) thereof project from the upper surface of the optical waveguide 200. This arrangement allows the filter member 206 to be handled with ease when the filter member 206 is inserted into the slit 204, because the slit 204 generally has a depth of several hundred μm.
However, since the upper portion of the filter member 206 projects from the upper surface of the optical waveguide 200, the following problems tend to arise:
(1) The gap between the slit 204 and the filter member 206 is filled with a resin 212. If the resin 212 poses a large stress (i.e., if it is a hard resin), then the stress is applied to the projecting portion of the filter member 206, tending to break the projecting portion of the filter member 206. This is liable to cause a reliability problem.
(2) The above problem (1) manifests itself if the slit 204 is formed obliquely within the optical waveguide 200, as shown in FIG. 10. Particularly, the portion of the filter member 206 that forms an acute angle with respect to the upper surface of the optical waveguide 200, i.e., the portion of the light-exiting surface 210 of the filter member 206 that projects from the upper surface of the optical waveguide 200, is subject to stress concentration and tends to be broken.
(3) If the gap between the slit 204 and the filter member 206 is filled with a resin 212 of low viscosity, then when the filter member 206 is inserted into the slit 204 with the upper portion thereof projecting, the filter member 206 itself serves as a guide, which allows the resin 212 to creep onto the upper surface of the optical waveguide 200, as shown in FIGS. 9 and 10. In this case, the refractive index of the resin 212 changes the effective refractive index of the optical waveguide 200, thereby affecting the propagation characteristics of the signal light.
(4) When the resin 212, which has crept onto the upper surface of the optical waveguide 200, is subsequently expanded or contracted, it applies stresses to the projecting portion of the filter member 206, tending to break the filter member 206.
(5) If a photodetector is mounted on the optical waveguide 200, then when the resin 212 that has crept onto the upper surface of the optical waveguide 200 enters the optical path of divided light, the light detecting characteristics of the photodetector with respect to the divided light become degraded, and the detecting accuracy thereof is lowered. This problem can be ignored if an optical fiber is used instead of the optical waveguide 200. However, if an adhesive made of a material different from the resin 212 is used to install the photodetector, then since a boundary occurs, which causes a refractive index change, the above problem cannot be ignored.
(6) If a photodetector is mounted on the optical waveguide 200, as described above in (5), then the upper portion of the filter member 206, which projects from the upper surface of the optical waveguide 200, tends to present an obstacle to proper mounting of the photodetector.